The present invention concerns a barium titanate based dielectric ceramic composition as well as a capacitor using this composition. It concerns, more particularly, a ceramic composition having a low relative variation of capacity within a wide temperature range, thus allowing said composition to reach class BX.
The article in the journal FERRO ELECTRICS, entitled "Electrode and materials problems in ceramic capacitors", by W. R. BUESSEM and T. I. PROKOROWICZ published in Volume 10--pages 225 to 230--1976 describes barium titanate compositions having a low sintering temperature using silver-palladium type electrodes that are far less expensive than electrodes made of gold or palladium. More especially, FIG. 6 of this article shows the results of measuring the relative variation of capacity with temperature for a dielectric having a constant equal to 1450. The variation curve under a voltage of 19.7 kV/cm, i.e. 2 volts/micron indeed corresponds to class BX, which means that the capacity variation of the said dielectric used in the form of a baked sheet of 25.mu. under 50 volts (nominal voltage) clearly meets the requirements of this class of products. However, the article points out simply that this composition is prepared on the base of barium titanate, glass and non specified additives. U.S. Pat. Nos. 3,619,220, 3,682,766 and 3,811,937, to which said article seems to refer with respect to the said compositions, show that the latter are prepared on the base of glass containing essentially cadmium oxide (about 33%) as well as bismuth oxide and lead oxide (25% by weight for each oxide). Among the possible additives to be used, it appears that neodymium oxide and lanthanum oxide are suitable.
However, as clearly demonstrated, by the article herein-above not all these compositions meet the requirements of class BX and those that do meet these requirements are not described therein. Furthermore, those compositions that meet the said requirements have a comparatively low dielectric constant.
The present invention is aimed at providing compositions that do not present this drawback, whilst having an excellent behaviour under varible temperature conditions in the absence as well as in the presence of an external electric field, so that some of said compositions will meet the requirements of class BX. (This class of components is defined especially by a relative variation of capacity .DELTA.C/C between -55.degree. C. and +125.degree. C. lower than or equal to .+-.15% for a signal of IV at the frequency of 1 kHz and by a relative variation of capacity lower than +15% and -25% in the presence of an external direct electric field of about 50 volts. Precise definitions can be found in U.S. Standard MIL C 11015 D.
With this object in view the invention provides compositions comprising a mixture containing 80 to 94% by weight barium titanate, 1 to 3% by weight neodymium oxide, 0 to 4% bismuth oxide, 2 to 5% by weight bismuth titanate and 2 to 8% glass frit.
According to a preferred embodiment, the compositions according to the invention contain 1 to 4% by weight bismuth oxide.
As will be seen herein-under, it has been established that such compositions (known by those skilled in the art as "type III compositions"), not only have a lowered sintering temperature as compared to standard sintering temperatures of barium titanate based compositions, but that surprisingly the said compositions also have excellent electrical properties within a very wide temperature range.
Preferably, non stoechiometric barium titanate is used, having a TiO.sub.2 /BaO molar ratio comprised between 1.06 and 1.09.
According to a preferred embodiment, the compositions according to the invention comprise more than 1.4% by weight, and preferably 1.4% to 2% by weight neodymium oxide. It has indeed been established that, surprisingly, under otherwise potential conditions, an amount of neodymium oxide higher than 1.4% by weight of the total composition allows compositions to be obtained which fully meet the specifications of Standard BX, more especially at low temperatures. But it has also been established that the increase in concentration of the neodymium oxide tends to amplify the relative variation of capacity at high temperatures.
The glass frit used within the scope of the present invention can have different compositions. The glass frit is obtained by a method well known per se, i.e. by mixing different ingredients used for producing glass, then melting these ingredients at high temperature, cooling to ambient temperature in order to obtain a glass that is thereafter ground into very fine particles, this powder being known as "glass frit". It has been established that it is necessary to use glass frit containing at least 70% by weight bismuth oxide, the other components being lead oxide and/or boron oxide. A fully satisfactory composition contains substantially 81% by weight bismuth oxide, 17% lead oxide and 2% boron oxide.
In order to further improve the electric properties and/or lower the sintering temperature of these components, up to 2% by weight of standard additives well known per se can be added. More especially, zinc oxide and/or manganese oxide can be added, as well as various mixtures thereof preferably in a proportion of about 0.5% by weight.
The invention also concerns multi-layer ceramic capacitors comprising electrodes containing 70% by weight silver, the 30% remaining being constituted by palladium. These capacitors are obtained according to the well known technique, comprising depositing electrodes by serigraphy on the dielectric layer and superimposing the layers thus produced, the even number electrodes and the odd number electrodes being respectively connected to each other, and the thus prepared assembly being then sintered at high temperature.